I live in southern Ontario Canada. Each spring, I purchase a new agapanthus, assuming that they will not survive our cold/snowy winter. Is this correct? If so, is it possible to bring it inside over the winter, and then return it to the garden in the spring? What sort of care would it require while inside?

Agapanthus are hardy to Zone 8 so they probably would not survive your winters. They don't survive at my house where temps don't usually drop below about 5*.

Most people who grow them in cold areas grow them in containers and bring them inside for the winter.

50 years ago I used to live in Flagstaff, AZ. I think it was zone 6B. They were hardy in the ground there, and came back every year. I don't know if it's still the case. Maybe leave one in the ground, and protect the rest during winter.

If you can keep it inside in a sunny window, it will stay green and make a really nice foliage plant. Your gentle sun and short days will help to keep it dormant through your winter. If you turn your thermostat down at night and/or shut off the heat vents near the plant to keep it cool it will be very easy care. Mine stay outdoors year 'round but don't grow through the winter, even though we get days in the 80's. They need very little water and no fertilizer until the weather is warming up in spring.

I would bring it in before the first frost, and wouldn't put it outside again until the night temperatures are dependably above 50F or so. ( that's 10C in Canada) It will get bigger and bloom more generously for you for years. They like to be root bound and don't like re-potting so just leave it in one pot as long as you can.